1. Field of the Invention
This invention relates to a projection lens for a television projector, and particularly to a projection lens used for a three-tube television projector.
2. Description of the Prior Art
In the television projector, an image plane of a cathode-ray tube (CRT) is enlarged and projected on a screen by a projection lens, but in the three-tube system, image planes of individual CRTs having respective fluorescent bodies of blue, green and red are superposed and projected on a screen by three lenses to obtain a color image. Therefore, a projection lens used therefor need not be achromatized.
Such lens systems composed of plastic lenses have been known from Japanese Patent Application Laid-Open Nos. 124114/80, 34515/82, 108818/82, 177115/82, etc., and large-aperture projection lenses which comprise less number, say three, and whose F number is 1.0 to 1.3 are obtained.
On the other hand, a television projector apparatus is required to have a large screen, but the whole apparatus has to be made smaller for popularization.
Therefore, in an optical system of the projector apparatus, a plane reflecting mirror is arranged between a projection lens and a screen to bend a light path to make the apparatus smaller. It is also proposed to make the apparatus further smaller that an in-mirror lens accommodating therein a plane mirror is used as a projection lens.
Among the aforementioned well known lens systems, there is one that a plane reflecting mirror is inserted between a lens on the side of the screen and an intermediate lens, among three or four lenses, the lens system capable of being used in the state of a large aperture and a wide angle of view. (See, for example, Japanese Patent Application Laid-Open No. 198017/83) However, no arrangement has been found wherein a plane reflecting mirror is inserted between a lens on the side of CRT image plane and an intermediate lens, and wherein a plane reflecting mirror is inserted both between a lens on the side of a screen and an intermediate lens and between an intermediate lens and a lens on the side of CRT image plane, for use.
In Japanese Patent Application Laid-Open No. 174297/83, which comprises three lenses, aberration is well compensated despite large aperture and wide angle of view while providing a large axial air space between the lenses to render the insertion of a reflecting mirror possible but the external diameter and axial thickness of the second lens are extremely large. Therefore, there was a problem in that if the second lens is composed of a plastic lens, a cycle time at the time of molding is extremely long and increases in cost as compared with a spherical glass lens of the same kind. In addition, since a rate of distribution of refractive power of the whole lens to the second lens is great and therefore, introvert comatic aberration remains at a portion where the angle of view is large, which has not always been satisfactory.